(a) Field of the Invention
The invention relates to an optical projection system and, more particularly, to a digital light processing (DLP) projection system and a projection method of the same.
(b) Description of the Related Art
The Digital light processing (DLP) technique has been developed by Texas Instrument since 1987. In a DLP projection system, an optical element called digital micro-mirror device (DMD) is utilized. On the digital micro mirror device there are thousands of micro-mirrors for reflecting light beams onto a screen to form an image. Each of these micro-mirrors has independent driving electrode, support post and steering shaft. When the DLP projection system is under a light state, light beams projected onto the micro-mirrors are reflected into a projection lens by these micro-mirrors and then focused on a screen. When the DLP projection system is under a dark state, these micro-mirrors are driven by applying a voltage to the driving electrodes and tilted to a specific angle, so that light beams projected onto the mirrors are reflected to other directions instead of reflected into the projection lens.
Referring to a usual DLP projection system 10 shown in FIG. 1A, when the projection system 10 is under a light state, inhomogeneous light beams from a light source are first converted into homogeneous incident light beams 17 via an illumination device 11, and then enter a prism set 12. The light beams 17 are totally internal reflected onto micro-mirrors (not shown) on a DMD 13, reflected into a projection lens 14 by these micro-mirrors, and projected onto a screen 16 in order. On the other hand, when the DLP projection system 10 is under a dark state, the micro-mirrors (not shown) on the DMD 13 deflects the incident light beams 17, so that the incident light beams 17 are deviated away from an optic axis of the projection lens 14 after passing through the prism set 12. To avoid unwanted light beams from entering the projection lens 14, a projection lens optical stop 15 is used for shielding deviation light beams 18 and stray light beams 19 coming from the light beams 17. In this DLP projection system 10, the design of the optical stop 15 is a crucial factor that affects contrast.
However, the prior DLP projection system 10 has certain drawbacks. First of all, since the prism set 12 reflects the homogeneous incident light beams 17 before they are projected onto the DMD 13 under a light state, chromatism is likely to happen due to dispersion effects of the prism set 12, which is not easily to be eliminated. Secondly, in order to ensure that the deviation light beams 18 and the stray light beams 19 would not enter the projection lens 14 under a dark state, a certain distance kept between the prism set 12 and the projection lens 14 is necessary, which, however, increases back focal distance and thus lowers image quality. Thirdly, adding extra optical stops to shield the deviation light beams 18 and the stray light beams 19 to provide a total dark state is an ordinary business. However, excessive optical stops may lower lumen of the projection system under a light state.
FIG. 1B shows an optical system disclosed by the U.S. Pat. No. 5,604,624, in which a conception of reflecting light beams from reflecting mirrors of a DMD to other directions using a second air gap under a dark state is revealed.
Referring to FIG. 1B, under a light state of a projection display optical system 10a, the inhomogeneous light beams coming from a light source pass through an illumination device 11 and are converted into homogeneous incident light beams 17, and enter a prism set 12a. The homogeneous incident light beams 17 are totally internal reflected at an interface between the prism set 12a and a first air gap 121, and projected to micro mirrors (not shown) on a DMD 13. The incident light beams 17 are then reflected into a projection lens 14 by these micro mirrors and further projected onto a screen 16. Whereas, when the projection display optical system 10a is under a dark state, the micro mirrors (not shown) on the DMD 13 deflect the incident light beams 17, such that the incident light beams 17 are totally internal reflected at an interface between the prism set 12a and a second air gap 122 and then deviated away from an optical axis of the projection lens 14. Also a projection lens optical stop 15 is utilized for shielding deviation light beams 18a and stray light beams 19a to prevent these unwanted light beams from entering the projection lens 14.
However, when this prior system is under a dark state, it is likely that part of the stray light beams 19a from the deviation light beams 18a still enter the projection lens 14. Hence, it is necessary to maintain a certain distance between the prism set 12a and the projection lens 14 or to provide an additional optical stop. As a result, a back focal distance of the system would be large and lumen of the system would be lowered. Under such circumstances, image quality and contrast of the projection display optical system 10a would be inevitably affected.
Therefore, the present invention provides a digital light processing (DLP) projection system in order to overcome issues and difficulties in the prior art. Without affecting brightness of the system, the DLP projection system according to the invention has a shortened back focal distance and effectively solves the stray light issue.